Titanium is an inert, metallic element having a high strength-to-weight ratio. Titanium has a relatively high melting point (1668.+-.5.degree. C.), which makes it particularly useful for high-temperature applications where other alloys, such as aluminum and magnesium alloys, fail. Titanium also has been used to produce high-strength alloys. These alloys are particularly useful for forming structural devices and ballistic armor.
These and other applications continually demand the development of new alloys. This generally is accomplished by modifying the composition of existing alloys, changing known processing regimens, or developing entirely new alloys and methods for their manufacture. However, it is difficult to predict how best to produce new alloys having desired properties. Small amounts of alloying materials and/or impurities can significantly alter the physical characteristics of the alloy, as can changing how the alloy is processed. For example, minor amounts of impurities can significantly increase the brittleness of the alloy. Kirk-Othmer's Concise Encyclopedia of Chemical Technology, pages 1182-1184 (John Wiley & Sons, 1985).
Titanium alloys, processes for their manufacture and devices made from titanium alloys also have been patented. U.S. Pat. No. 5,332,545, entitled Method of Making Low Cost Ti-6Al-4V Ballistic Alloy, describes a process for providing equivalent or superior ballistic resistance performance compared to standard Ti-6Al-4V alloys. The process requires increasing the oxygen content to be greater than the conventional limit of 0.20% maximum. The oxygen-rich alloy is then heated at temperatures within the .beta.-phase field, which is referred to as .beta. processing. The '545 patent teaches avoiding .alpha.-.beta. processing because it allegedly causes cracks in the alloy, and because it generally is more expensive than .beta. processing.
U.S. Pat. No. 5,435,226, entitled Light Armor Improvement, describes a structural armor assembly. The assembly includes a superplastically formed sandwich arrangement that includes a high-toughness, high-strength titanium alloy material. The titanium alloy includes 4.5 weight percent aluminum, 5 weight percent molybdenum and 1.5 weight percent chromium.
Chakrabarti et al.'s U.S. Pat. No. 4,898,624 concerns Ti-6Al-4V alloys which are processed to obtain desired microstructures. Chakrabarti's alloy has 5.5-6.75% aluminum, 3.5-4.2% vanadium, 0.15-0.20 weight percent oxygen, 0.025-0.05% nitrogen and 0.30% iron. The processing steps comprise preheating the composition above the .beta. transus temperature, followed by rapid cooling.
Eylon et al.'s U.S. Pat. No. 5,032,189 concerns .alpha.-.beta. alloys. A primary object of Eylon is to provide a new method for forging known near-.alpha. and .alpha.+.beta. titanium alloys. The alloy processing steps comprise forging an alloy billet (a billet is a bar or ingot of a metal or metal alloy in an intermediate processing stage) to a desired shape at a temperature approximately equal to the .beta.-transus temperature of the alloy, cooling the component, annealing the component at a temperature about 10 to 20% below the .beta.-transus temperature, and cooling the component in air.
Despite the titanium alloys previously developed, there still is a need for additional alloys, particularly for specialized applications such as ballistic armor.